The present invention generally relates to electronic circuits, and more particularly, to a start-up circuit for a current-controlled oscillator.
Most digital circuits use clock signals to synchronize various functions performed by the digital circuits. To generate the clock signals at desired oscillating frequencies, the digital circuits typically employ a clock generating circuit such as a current-controlled oscillator (CCO). The CCO includes series connected inverters that are controlled by a driving current for generating an output voltage at a desired oscillating frequency, where the oscillating frequency is directly proportional to the driving current. For example, when the driving current is high, the oscillating frequency is high, and when the driving current is low, the oscillating frequency is low.
The CCO includes a start-up circuit that generates the driving current. Existing start-up circuits include a high-gain reference circuit and a current generator. The current generator supplies a start-up current to the high-gain reference circuit, which in turn amplifies the start-up current. The high-gain reference circuit also generates the start-up current used to control the oscillating frequency of the output voltage of the CCO.
Digital circuits usually operate at a high clock speed (typically in mega-Hertz), and the oscillating frequency of the output voltage should reach the desired oscillating frequency within a short time period (typically in microseconds). Further, the CCO needs to generate the output voltage at the desired oscillating frequency within this time period to sample data correctly. Thus, to achieve the desired output voltage oscillating frequency, the driving current must reach a desired current within the short time period. However, the high-gain reference circuit cannot generate the driving current until the current generator supplies the start-up current to it, which impacts the start-up time of the ring CCO. Further, when the current generator supplies the start-up current, the high-gain reference circuit may generate a driving current that is less than desired. Hence, the CCO may fail to generate the output voltage at the desired oscillating frequency.
Conventional start-up circuits also employ current generators to generate a driving current that is greater than the desired current within the short time period. However, if the driving current overshoots the desired current, it causes the output voltage oscillating frequency to be greater than the desired oscillating frequency, which can cause the clock signal to incorrectly sample data. Hence, there is a need for a CCO with a start-up circuit that accurately generates the driving current for driving the CCO so that the CCO generates the output voltage at the desired oscillating frequency, and also has a faster start-up time.